The invention relates to an X-ray device which includes an X-ray source and a sensor matrix that is covered by a scintillator, and also includes means for measuring the X-ray dose incident on the sensor matrix.
For the safety of the patient as well as for the image quality it is necessary to measure and control the X-ray dose in X-ray diagnostics. Among the means for measuring the X-ray dose there are means that operate on the basis of preceding images and those that measure the X-ray dose in real time.
EP 0 779 521 A1 discloses an X-ray device with an X-ray source and a sensor matrix that is covered by a scintillator, as well as with means for real-time measurement of the X-ray dose incident on the sensor matrix. The means for measuring the X-ray dose (dosimeter) consist of a matrix of rectangular electrode elements which are arranged so as to be separated from a further electrode layer by an amorphous silicon layer. The silicon layer converts the X-rays into electric signals and behaves as an ionization chamber in which ions are formed that are charged in proportion to the intensity of the X-rays. The number of ions forms an electric signal corresponding to the X-ray intensity.
The signals delivered by the detection elements of the dosimeter, being constructed as X-ray sensors, are very small and hence difficult to evaluate. Moreover, the X-ray sensors degrade the image quality.
On the basis of the described state of the art the invention has for its object to avoid said drawbacks.
This object is achieved for an X-ray device of the kind set forth in that a matrix of light-sensitive detection elements is arranged over the imaging sensor matrix and in the path of the X-rays and that a partly transparent reflector is arranged between the scintillator and the matrix with detection elements, said reflector being connected to the scintillator and to the matrix with detection elements and the number of sensors of the sensor matrix being larger than the number of the detection elements related to the relevant matrix surface.
The construction of the sensor matrix, notably an FDXD (Flat Dynamic X-ray Detector), is that of a customary FDXD. The pixels of the sensor matrix customarily consist of a respective photodiode and a TFT transistor and are arranged in a matrix consisting of rows and columns. The scintillator converts the X-rays incident on the sensor matrix into visible light. The light is absorbed by the photodiodes of the individual pixels of the sensor matrix that are situated therebelow and is stored as an electric charge in the capacitances of the photodiodes whose charges are periodically read out via thin film transistors (TFTs). The reading out progresses from row to row. The scintillator is customarily provided with a reflector which serves to reflect the visible light that is reflected by the scintillator directly back into the matrix so as to optimize the signal strength. Now the major difference between the invention and the conventional X-ray devices with a dosimeter commences. According to the invention a partly transparent reflector is connected to the scintillator. This partly transparent reflector allows a small part of the light to emanate again from the scintillator so as to enter the matrix with light-sensitive detection elements that is arranged above the sensor matrix.
The number of sensors of the sensor matrix exceeds the number of detection elements, that is, related to the relevant matrix surface. This means that the detection elements have a surface area that is larger than that of the sensors of the sensor matrix, for example, 1xc3x971 cm2 instead of 0.15xc3x970.15 mm2. When the significantly smaller number of light-sensitive detection elements is read out with high frame rates during an X-ray pulse, the device in accordance with the invention enables real time information concerning the signal build-up in the detection elements during the X-ray pulse to be used so as to control the dose of the instantaneous X-ray pulse. Because only approximately 5% of the light from the scintillator is required for the light-sensitive detection elements, the greater signal strength in the sensor matrix results in a significantly enhanced image quality in comparison with the state of the art. The partly transparent reflector, moreover, ensures that a fixed relationship exists between the dose signal for the light-sensitive detection elements and the image signal for the sensor matrix, said relationship being defined by the degree of partial transparency of the reflector and being, in a technical sense, practically independent of the spectral composition of the X-rays. The spectral composition is determined inter alia by the anode material of the X-ray source, the material and the thickness of the exit window for the X-rays at the X-ray source, the high voltage used and the filtering and absorption of the X-rays by the patient and the table.
In order to minimize the absorption of X-rays, the substrate material of the matrix with detection elements in a preferred embodiment of the invention consists of a film of a synthetic material that has a low X-ray absorption, for example, polystyrene, polyester or polycarbonate.
When the light-sensitive detection elements are constructed as photodiodes provided with an amorphous silicon layer, no damaging of the detection elements by the X-rays is to be expected. The metallic electrodes of the photodiodes, however, could absorb a small part of the X-rays and give rise to image defects in given circumstances. In order to avoid such image defects, in an embodiment of the invention it is proposed to provide the matrix with light-sensitive detection elements with a transparent metal layer that covers those regions of the matrix surface that are not covered already by transparent metal layers formed by the electrodes of the detection elements. The different metal layers together cover the entire matrix surface so that image defects are precluded.
In order to enable effective influencing of the dose of the X-ray pulse already during the signal build-up, it is advantageous to read out the matrix of the detection elements with high image repetition rates. The image repetition rates preferably amount to from one thousand to some tens of thousands of images per second. In any case the image repetition rate should be selected to be so high that in the case of an X-ray pulse duration of only a few milliseconds an adequate number of read-out operations can be performed in the matrix with detection elements so as to enable adequately accurate adjustment of the desired dose.
The read-out speed can be increased by reading out in parallel a plurality of rows of detection elements that are also arranged in rows and columns. Finally, from the matrix of detection elements sub-regions can be selected for the measurement and control of the X-ray dose. For example, those regions that are exposed directly to the X-rays can be selected.